4.5.3 Recording of distance travelled
As well as indicating ship's speed, logs record and display distance travelled. It is good navigation practice to initialise the log distance trip at the start of each new track, and record log distances in the logbook at the end of each watch.
4.6Echo sounders
The navigational echo sounder should be expected to operate down to depths of at least 200m (approximately 110 fathoms).
The echo sounder should always be used when making a landfall and kept switched on in coastal waters. If the echo sounder is fitted with a shallow water alarm, the alarm should be set to an appropriate safe depth to warn of approaching shallow water.
Care should be taken to check that the units of soundings on the echo sounder are the same as those used on the chart in use. When comparing echo and chart soundings, allowance must be made for the draught of the ship, and any water stand or tidal effects.
4.7Electronic position-fixing systems
Electronic position-fixing systems provide an automatic and continuous position update for ships fitted with a suitable receiver using either a terrestrial hyperbolic radio navigation system such as Loran C, or a global satellite system such as GPS.
4.7.1Hyperbolic positioning systems
The use of hyperbolic positioning systems at sea is declining. Omega is no longer operational and Decca is being phased out. Loran C, as a back up to the global navigation satellite system, is to be retained for the time being.
The use of lattice charts showing hyperbolic lines of position has also declined, and most receivers convert the readings to latitude and longitude.
4.7.1.1 Loran C
Loran C has a basic range of approximately 1200 miles using ground-wave signals, although extended range coverage is possible using skywaves.
Corrections need to be applied to Loran C signals to take into account variations in the conductivity of the earth's surface over which the signals pass. These are known as additional secondary factor (ASF) corrections. The corrections may need to be manually applied before plotting the position on the chart.
4.7.2Global navigation satellite system
A global navigation satellite system (GNSS) is a satellite system that provides ships fitted with suitable receivers with a means of obtaining continuous worldwide position, time and speed information.
The Global Positioning System (GPS) operated by the United States and the Global Navigation Satellite System (GLONASS) operated by the Russian Federation are currently available for civilian use on ships.
4.7.2.1 GPS and DGPS
GPS offers commercial users a global positioning capability with accuracy of the order of 100 metres.
Differential GPS (DGPS) receivers apply corrections to raw GPS signals determined and transmitted by terrestrial monitoring stations. Differential signals can be transmitted to ships via satellites or using HF radio links. Within DGPS coverage, positional accuracy of the order of 10 metres at the receiver antenna is possible.
4.7.3Use of electronic position-fixing systems
Care should be taken when using electronic position-fixing systems.
Watchkeepers need to understand the capabilities and limitations of the systems they are using and continually monitor and validate the information given.
4.7.3.1Use of electronic position-fixing systems in integrated bridges
When position-fixing systems transmit data to other navigation systems, the integrity and quality of the data transmitted need to be safeguarded.
Techniques used should include:
•using pre-set quality limits to monitor the fix quality of each position-fixing system connected to the integrated bridge;
•comparing all positions to identify and reject any rogue positions or positions that are clearly incorrect;
•comparing electronic positions with the ship's estimated position (EP) calculated using direct inputs from the log and gyro;
•checking the status of the data transmitted and ensuring that only valid data messages are used.
4.7.3.2Route monitoring
Route storage and cross track error (XTE) monitoring are common GPS features. By entering the passage plan in the GPS as well as the navigation system, the GPS can provide an integrated bridge system (IBS) with an independent route monitoring capability.